1. Field of the Invention
The present invention relates to a printed circuit board having bumps and to a method of forming the bumps that serve for connection terminals in the connection portions and land portions of the printed circuit board such as an FPC. More specifically, the invention relates to a printed circuit board having uniform and tall bumps and to a method of forming the bumps formed by applying a plating onto the printed circuit board up to the height of the bumps followed by etching the plating.
2. Prior Art
The FPC is usually coated with a cover lay film. However, in order to prevent the terminals for connection and the land portions for mounting parts to be covered with the cover lay film, holes are formed in such portions of the cover lay film in advance using a drill or a punch prior to applying the film. This, however, makes it difficult to achieve the contact with the circuit pattern of the opposite side unless bumps for connection are formed in portions corresponding to the holes by some means.
Method of forming bumps have been disclosed, for example, in Japanese Patent Laid-Open Nos. 98186/1988 and 152147/1982.
According to the method disclosed in Japanese Patent Laid-Open No. 98186/1988, a flux is supplied into through holes connected to conductors of the board circuit, a solder ball is adhered on each through hole and is melted to form a solder protuberance at an opening of the through hole.
According to the method disclosed in Japanese Patent Laid-Open No. 152147/1982, metallic protuberances are formed on the board such as of a glass and is then transferred onto metal leads of the FPC.
There has further been proposed a method according to which corresponding portions of the circuit pattern of FPC are pushed up from the back side using a pin-like jig in order to form bumps.
In the method of adhering the solder balls, however, apertures must be formed prior to forming the through holes. The aperture usually has a diameter of 0.4 mm at the smallest. Further, to place the solder ball thereon, the bump must have a diameter of about 1 mm. Even under optimum conditions, it is not possible to reduce the size of the bump to less than 250 .mu.m in diameter. Because of this reason, this method could not be used for the FPC having fine patterns. It is further necessary to reliably place the solder ball on each through hole without deviation. Furthermore, the material of the bump is limited to the solder and the flux must be removed as well.
With the method of transferring metallic protuberances, it is difficult to form bumps having a height greater than 100 .mu.m. When the metallic protuberances are formed with gold plating, furthermore, the manufacturing cost increases. Furthermore, this technology must form bumps on the finger portion of a flexible tape in the TAB system for mounting semi-conductor elements on the flexible tape. In the case of the FPC in which the circuit pattern is coated with a cover lay film, therefore, it is difficult to transfer the metallic protuberances onto predetermined positions of the FPC. In the case of large FPC's, in particular, this becomes more difficult due to dimensional changes. Furthermore, owing to the thickness of the resist and so on, the height of the protuberances is about 20 .mu.m at the greatest.
With the method of pushing up the circuit pattern with a jig, furthermore, cracks develop when the circuit pattern is pushed up and reliability is lost. Furthermore, it only those bumps having a height smaller than the limit of plastic deformation of the Cu pattern that is are formed. In the case of the double-sided board having circuit pattern on both surfaces, it is not possible to arrange the Cu pattern on the back side of the bump-forming portion. This method, therefore, is used only for one-sided board. Further, it is not possible to reduce substantially the diameter of the pin that serves as a squeezing pin making it difficult to form bumps having diameters smaller than 250 .mu.m. When the conductor patterns at the connection portions are arranged maintaining a small pitch, it also becomes difficult to arrange the pins for pushing up.
Another method consists of coating the board with a cover film except the portions for forming bumps, and depositing a plating on the portions where bumps are to be formed. By taking the bulge of the adhesive into consideration, however, the hole in the cover film corresponding to the bump-forming portion must have a diameter of at least 0.3 mm. Furthermore, the plating may grow laterally by the thickness of the plating causing the bump to assume the shape of a mushroom. Even with this method, therefore, the bump tends to have an increased size; at present, it is difficult to form bumps having diameters of smaller than 0.2 mm. It can further be thought to cover the board except the portions for forming bumps by stacking several resist layers such as dry films to a thickness of about 150 .mu.m and then apply copper plating on the portions for forming bumps. However, the dry film has a thickness of 35 .mu.m at the greatest, and it is virtually difficult to advance the process while stacking several such dry films.